Frequency selective-current transmission



R. c. MATHES FREQUENCY SELECTIVE CURRENT TRANSMISSION Qpiginal Filed Dec. 17. 1921' 2 Sheets-Sheet 1 /vz/enfok' flober/ C. Maf/zes Dec. 28 1926.-

. 1,611,932 R. C. MATHES FREQUENCY SELECTIVE CURRENT TRANSM-ISS ION Original F iled Dec. 1'7.

.1921 2 Sheets-Sheet 2 Frequency 1: f5 Frequency f5 f5 Frequ en cy f {5 Frequency hvemor Robe/4 C/Va/hes Frequency 5y fay/ y Patented Dec. 28, 1926.

UNITED STATES PATENT OFFICE.

ROBERT C. MATHES, or WYOMING, NEW JERSEY, AssIGNoR r0 WEs'rERN ELECTRIC COMPANY, iNCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

FREQUENCY SELECTIVE-CURRENT TRA SMISSION.

Application filed December 17, 1921, Serial No. 522,976. Renewed June 8. 1926.

ted has been to use two or more sections of a low pass filter of the type shown in Fig. 7 of U. S. patent to G. A. Campbell, 1,227 ,114, May 22, =1917. The present invention aims -tO accomplish this object with cheaper forms of filters, and alsoaims to obtain a sharper cut-off than is obtained withthe two Campbell filter mentioned.

The attenuation of certain of these new forms of filters, though high in the neighborhood of their cut-off frequencies, falls off at higher frequencies to such an extent that it is desirable to supplement their filtering action. To obtain the desired supplemental filtering section action the transmission efficiency of the input transformer associated with the repeater may be made to fall off by 'a considerable amount at frequencies above the highest frequency to be transmitted, so that the total filtering effect of the transformer and the filter will substantially prevent the transmission of higher frequencies.

In accordance with the invention, in some cases a form of filter is employed which has sufiiciently high attenuation throughout the attenuation region to substantially prevent transmission over this frequency range regardless of the transmission efficiency of the transformer, and, of course, in such cases no filtering action of the.transformer need be relied upon. i j

A cheap filter with asharp cut-off is obtained by, in effect, employing a resonant, or

suppression, branch in the filter, fixing the resonant frequency at a. point near the cutof certain illustrative embodiments of the invention which are shown in the accompanying drawings wherein Fig. 1 is a diagram of a repeating system employing a low pass suppression type of filter giving the loss frequency curve of Fig. 7 and an input transformer giving the loss frequency curve of Fig. 6, the filter and the transformer together giving the loss frequency curve of Fig. 8 and therefore substantially preventing the transmission of frequencies higher than. the cut-off frequency of the filter; Fig. 2 is a diagram of a similar system usin a filter having two rather loosely coupled windings in the coil, and which gives the frequency curve of Fig. 10; Fig. 3 shows a filter with a similar coil, and which will sub stantially prevent the transmission of frequencies higher than its cut-off frequency without assistance; Fig. 4 shows a filter obtained by omitting the end condensersof the filter of Fig. 2; and Fig. 5 shows a filter which is externally electrically equivalent to the filterof Fig. 3, but which has no mutual inductance between the inductanccs' at one side of the middle shunt branch of the til ter and the inductances at the other side of that branch. I

In Fig. 1 a vacuum tube repeating element 1.2 is connected between lines14 and 16 by means of an input transformer 18 and an output transformer 20. A filter 2 2, consisting. of condensers 24, 25 and 26, and an inand transformer 20. Filter 22 is a low pass suppression type filter such as is shown in Fig. 10 of the U. S. patent of G. A. Campbell, No. 1,493,600, granted May 13, 1924. A battery B supplies space current for the tube through the primary Winding-Of transformer 20 and the inductance 27. A condenser 28 prevents the flow of direct current from the positive pole of the battery B di rectlytorthe negative pole of the battery. and by-passes the a. c. Output of the tube around thebattery.

The input transformer preferably has high mutual inductance and high leakage inductance, with a transmission loss frequency curve such as is shown in Fig. 6. This transformer may be, for instance, a transformer of the type disclosed in the application of F. E. Field, Serial No. 512,147, filed November 1, 1921, issued .as Patent No. 1,507,994, September 9, 1924, or may be of any suitable type.

Since the loss increases with frequency from the point i, to infinity, the transformer may be regarded as, in effect, a filter with a cut-off frequency f,, which should preferably be in the neighborhood of the highest frequency to be transmitted.

Filter 22 has the loss frequency curve of Fig. 7, with a cut-off frequency f, at the highest frequency to betransmitted and a frequency f =of extremely high attenuation (in fact infinitely high for an ideal resistanceless filter, due to resonance of inductance 27 with capacity at frequency Above frequency the lossfalls oft, however- The combination of Fig. 6 with Fig. 7 will give the curve of Fig. 8 if the cut-off frequencies of the transformer and the filter are made substantially equal. In Fig. 8 the minimum transmission loss outside of the transmission range is denoted by A. The

magnitude of this, loss in certain combinations tested is of the order of that of 15 miles of standard cable at 800 cycles per second The circuit of Fig. 1, embodying the transformer and the filter each with the characteristics described is suitable for use,

for instance, in 21 type telephone repeaters, and is cheaper than the two-section low pass Campbell filter of the type shown in the above mentioned Patent 1,227,114, since the filter of Fig. 1 employs only one inductance coil. Moreover, thefilter of F ig. 1 can be given a sharper cut-off than the two-section low pass Campbell filteromentioned, since the resonance frequency of loop 25,27. can be made close to the cut-off frequency.

The system of Fig.2 is like that of vFig. 1 except that insteadof the filter 22 a filter 30, having the loss frequency curve of Fig.

.9 is employed. Filter consists of two condensers 1/2 0,, a condenser 0 a' condenser 0 and a single inductance coil having two windings in separate ineshes of the filter and thus introducing mutual inductance M between these two meshes. The self inductance,- or leakage. inductance of each wind-' ing is designated L.' The two windings are i I preferably wound in series opposing relation on the core of the coil, and the coupling between the two windings is preferably con siderably less than unity. The reason why it is preferable that the windings be in series opposition rather than in series aiding relation will appear hereinafter.

- When the two windings of the filter 30 are wound in series opposition the filter has an attenuation region with two frequencies and f, of theoretically infiniteattenuation. This attenuation region is'followcd by a transmission region between 7", and f after which there is another attenuation region. In this second attenuation region the attenuation approaches a constant finite value as the frequency is increased. The combination of Fig. 6 with Fig. 9 will give the curve 'of Fig. 10 if the cut-off frequencies 7",, of the transformer and the filter be made substantially equal. In Fig. 10 the minimum transmission loss outside of the first transmission region is indicated by A, and can be made greater than the amount A shown in Fig. 8. The circuit of Fig. 2 is therefore preferable to that of Fig. 1 in 22 type repeaters with high gains. The filter 30, in addition to having a better attenuation characteristic, is cheaper than a two-section low pass Campbell filter of the two-element type shown in Fig. 7 of the above mentioned Patent No. 1,227,114. because the expense saved by the elimination of one coil is greater than that incurred by the addition of the condenser 0 Fig. 4 shows a filter 40 which may be used in a system such as that illustrated in Figs. 1 and 2, to supplement the filtering action of the transformer. This filter has no condenser-s corresponding to the end condensers quencies higher than its cut-off frequency without assistance. This filter has no condenser corresponding to the condenser 0,, of Fig. 2, and has a single coil, like that of Fig. 2.. Thus the filter of Fig. 3 employs only the same number of condensers asare required by the two-section low pass Campbell filter of the two-element type shown .in F ig. 7 of the Patent No. 1,227,114, and employs one coil less than is required by such Campbell filter. Moreover, the cut-off may be made sharper than that of such Campbell filter.-

Further, the grain characteristic of a repea-fer using the filter of Flg. 3 is more closely similar to the singing point characteristic of loaded cable with standard epeater networks, and therefore thetheoret-ical cutoff point of the filter can be placed somewhat higher than in the case where such Campbell filteris employed,'and thus-a further conlou tribution to the quality of transmission may be obtained. I

F 1g. 5 is merely for the'purpose of exductance L and the capacity C form one 'midseries terminated section of a three-element or suppression type low pass filter such as is disclosed in Fig. 9 of the U. S. patent to G. A. Campbell, No. 1,493,600, mentioned .abo've: oneof the inductances 1/2 L",, and its adjacent condenser 1/2 C" form one half section of atwoelement low pass Campbell. filter of the type shown in Fig. 7 of the Fat ent 1,227,114; and the other inductance 1 /2 L and capacity 1/2 C" form another half section of such type. The filter 'of 3 is externally electrically equivalent to the filter of Fig. 5 (although the former filter has an advantage over the latter in that by specially designing the core ofthe inductance coil in Fig. 3 to be free from hysteresis and eddy currents the maximum attenuation of the filter'may with practicability be made to more nearly approach infinity than can the maximum attenuation of the filter of r Fig. 5 which it is impracticable to make without a considerable amount of ohmic resistance, in the two-element shunt branch comprising the inductance coil'L for, as is explained in U. S. patent application of K. S. Johnson, Serial No. 434 388, filed December 31, 1920, a structure such as that consisting of the two self inductances L, having mutual inductance M, and the condenser C of Fig. 3 is equivalent to a structure such as that consisting of the two inductances LM, the inductance L (:M)" and the condenser C (:C) of Fig.5.

Let k be the coupling factor of the coil of Fig. 3; left 7, be the frequency of maximum attenuation or the resonant frequency of the two-element shunt branch of the filter section of Fig. 5 which consists of the two inductances 1/2 L the inductance L and the condenser C let f be. the cut-off and let' i I7 IT Z E E C C approaches zero. Then the design formulae for. the filter of Fig. B'may be given as follows: Y Y

a 1 a1:- Z

, Tests haveshown-I the actual loss curve for a filter of the type shown in Fig. 3 to agree quite closely with the theoretical curve.

One filter which, upon test, checked this relation has a small toroidal inductance coil with the two windings wound on spools on sponding value of Z was 504 and the filter was used between impedances of 500 ohms each. The loss curve for this particular filter does not fall below a loss for 24 miles of standard cable at 800 cycles per second in the attenuated region, and the'impedance curve is not very different from ith tlt found foma low pass Campbell filter'like Fig. 7 of the Patent No. 1,227,114. lhecondensers 'ofthis Campbell filter were of the same values as those. given above for the end condensers of the filteixof the tvne of Fig. 3;

the middle condenser of the latterfilter is slightly smaller than either end condenser; and the filter employs only one inductance .coil instead of the two coils required for two sections of the Campbell filter. Consider the filter of the type shown in F p1 'fu 1 2 72: 2 3 7 3 p that is, let p z2vrf Then the boundaries of the region of unattenuated transmission are given by the equations clp+ 4 i (1) readily reduces to the solutions of which are (1") reduces to which is (IT-211E]: F

It is important to determine the relative I positions of the band edges given by equations (2) -(5). Let us write Then we have Thus is always greater than for all positive values of k 12 is greater than p flfor positive is and with most choices of capacities p, is greater than 77 v \Vhen 7a:+1, p andp v become infinite. lVhen 7c:1, p, becomes infinite.

' Itshould be noted that none of the-band edges can ever become imaginary.

It can be shown that the attenuation becomes infinite when Thus there are in general two frequencies the equations of infinite attenuation. These are given by The two fre ucneies are real and distinct, real and coincident, or imaginary according as is greater than, equal to, or less than zero.

It has been found by trial that with numerical values of is ordinarily used .these frequencies are less likely to be imaginary if the halves of the coil are connected in series opposition than if the halves of the coil are connected in series aiding relation. Therefore, as has been pointed out above, it is preferable that the seriesopposition connection be employed.

Vhen Z:- Z the expressions for the frequencies of infinite attenuation become Let Z be the low frequency iterative impedance at mid shunt, that is, the iterative impedance at mid shunt when the frequency approaches zero. Then it can be shown that 0 O d-C2 It is noted that when the frequency is 0,

the impedance of ,condenserc becomes infinite, and then the filter becomes thee'quivalent of the filter of Fig. .3.

In the design of a filter of the type shown in Fig. 2 there are in general, five constants to be fixed; namely L, M (or k), 0,, 0 0 Expressions involving these constants for six quantities which are all important in their relations to the characteristics of the filter have been given above. These quantities are P2, 2 3 2 4, it, 2 6 O- y Choosing five of them arbitrarily the elements-of the filter can be determined.

Now in most cases the coeflicient. of coupling between the halves of the coil must be fixed, at least approximately, by considerations of coil manufacture rather than by the characteristics of the filterf Hence there are only four constants L, 0,, c 0 to determine by the design formulae, and so only four of the quantities 7) 7),, 77,, 77 19 Z can be chosen arbitrarily. Z and p, are always of great importance, and consequently one must be able to fix them at will. \Vhich other two of the quantities one may choose to fix arbitrarily mustbe more or less decided by circumstances.

.If p, and 7),, were chosen the design formulae would be extremely complicated. A simple set of design formulae are obtained if 72 p 7), and Z are fixed. We thus obtain 2])40. I 102( (z 2 P4 VQh ##W values of L, M, 0 and 0 as in the general case. By reducing equations (12) the fol-' lowing design formulae may be obtained:

What is claimed is: I

1. A. system for transmitting a band of frequencies with hi h efficiency, said-system comprising a trans ormer having a substantially uniform transmission loss over said band and a substantial slope in its lossfrequency characteristic at an edge of said band, and said system comprising a frequency selective circuit serially related to said transformer, said frequency selective circuit having in its loss frequency characteristic at said edge of said band'a substantial slope of the same sign as said. first mentioned-slope.

Ordinarily p lies below 9 As the 2. A frequency selective transmission system comprising a wave filter and a transformer in serial relation, said filter including series and shunt arms, each conqirising lumped reactance, said filter having a cutoff frequency, and said trmist'ormer having in its transn'iission frequency characteristic in the neighborhood of said cut-off freipiency a substantial slope of the same sign as the slope of the transi'nission freqlu-mcy characteristic of the filter at said cut-ofi' frequency. 3. A'fre'quency selective transmission sys tem comprising a transformer and a Wave filter in serial relation, said filter having high attenuation over a frequency range over which the transmission loss in said transformer is low and said. transformer having high transmission loss overa frequency 2 range over which the attenuation in said filter is low, whereby the resultant attenuation in said transformer and said wave filter is high over both ranges.

4. Afrequency selective transmission system 'comprising a transformer and a wave filter in serial relation, said filter having high attenuation over a frequency range over which the transmission loss in said transformer is low and said transformer having high transmission loss over a frequency 'ange over which the attenuation in said filteris low and which lies within the attenuation region of said wave filter.

5. A frequency selective transmission system comprising .a wave filter and a. transformer serially related, said filter having a finite critical frequency and an attenuation region and having lower attenuation at a point in sald reglon remote from said frequency than at said critical frequency, andsaid transformer having high attenuation at said point.

6. A frequency selective transmission system comprising a filter having a cut-off frequency and, serially related to said filter, a

transformer having an attenuation frequency a characteristic different from that of said filter, but with a frequency range of high 7 attenuation having a frequency limit \stantially said cut-off frequency.

, 7 Afrequency selective transmission system comprlsing a Wave filter having a frequency range of substantial attenuation, and an attenuation frequency characteristic with a sloplng portion at an edge of said range,

and serially related to said filter, a trans former having in its attenuation frequency characteristic over the frequency range of said sloping portion a substantial slope of the same sign as that of said portion,wliereby the composite attenuation frequency characteristic of said filter and said transformer has a sharper cut-off than the characteristic of said filter.

8. A frequency selective transmission system comprising a transformer having frequcncy ranges ofhigh and of low attenuation, and having a transmission frequency characteristic with a substantial slope beginning at a given frequency and, in serial relation to said transformer, a wave filter, said wave filter having a critical frequency substantially. the. same as said given fre quency, and also having a frequency of maximum at-tendation close to said given frequency and lying within the high attenuation region of said transformer. v

9. A frequency selective transn'iission system comprising a wave filter having a region of high attenuation with a cutfolf frequency at one side of said region, the attenuation in said filter being low at a frequency lying without the free transmiss'on region of said filter and remote from said cut-off frequency, and, serially related to said filter. a transformer havinglow attenuation without and at said one side of said high attenuation region and having high attenuation without and at the other side of said high attenuation region.

10. A frequency selective transmission circuit comprising a transformer and awave filter serially related, said filter being adapted to be connected between two sections of said circuit and comprising an impedance elements, an inductanceat each side of said impedance element, and condensers connected across said sections at the ends of said inductances, said inductance s being so related as to have mutual inductance, said filter having a point of maximum attenuation near its cut-off frequency and having lower attenuation ata frequency higher than that of said point, and said transformer having low transmission loss at and below said cut-off frequency and having high transmission loss at said higher frequency.

11. A frequency selective transmission circuit comprising a transformer and a wave filter seriallyrelated, said filter comprising two pairs of terminals for connection to said circuit, shunt paths including condensers between said pairs of terminals, inductances at both sidesj'of one of said shunt paths, and a V condenser connected across said inductances,

an inductance at one side of said shunt path haying mutual inductance with an inductance at the opposite side, said filter having a cut-off frequency and having low attenuation at a frequency higher than said cut-off frequency, and said transformer having low transmission loss at and below said cut-off frequency and having high'transmission loss at said higher frequency.

12. A frequency selective repeating system comprising a repeating element, an input transformer therefor, and a wave filter seriallv related to said transformer. said filter having a finite critical frequency and an at tenuation region and having lower attenuation at a point in said region remotefrom said frequency than at said critical frequency, and said transformer having high transmission loss at said point. I

13. The method of frequency selective transmission which comprises transforming a group of components of a complex current wave which have certain frequencies with lower efficiency than the eliicicncy with which components having other frequencies are transfornu-d, and increasing the sharpness of discriuiinalion against said group atv one limit of said group bv suppressing, to a high degree, a comparativclv narrow band of frequencies adjacent said limit. and lying within said group.

14. The method of frequency selective repeating which comprises transforming a group of components of a complex current wave which have certain frequencies with lower efiicicncy than the ctiiciency with which components having other frcquenciei are transformed, repeating the transformed and distorted wave, and suppressing, to a higlrdegree, a comparatively narrow band of the repeated wave components which is adjacent said limit and which lies within said group.

15. In combination, a filter capable of freely transmitting all frequencies below a superior limit and substantially attenuating frequencies above said'limit, and, serially related to said filter, a transformer capable of substantially attenuating frequencies above and inthe neighborhood of said limiting frequency, and freely transmitting frequencies below and in the neighborhood of said limiting frequency.

16. An electrical network comprising a condenser, means for connecting said network between two sections of a circuit, an inductance between said condenser and each of said connections, and a capacity connected across said inductance, said inductances being s0 related as to have mutual inductance. 17. An electrical network adapted to be connected between two sections of a circuit, saidnetwork comprising a shunt condenser, an inductance at each side of said condenser, and a capacity connected across said inductances, said inductances being so related as to have mutual inductance.

18. An electrical network adapted to be connected between two sections of acircuit, said network comprising an impedance element, an inductance at each side of said impedance element, and a capacity connected across said inductances, said inductances being so related as to have mutual inductance.

19. A filter section having two pairs of terminals for connection to a line or other filter sections, a shunt path comprising a condenser between said pairs of terminals, inductances at each side of said shunt path, and a condenser connected across said inductances, an inductance at one side of said I shunt path being wound on a common core in series opposingrelation with an inductance on the opposite side.

20. An electrical network adapted to be connected between two sections of a circuit,

said network comprising an impedance element, an inductance at each side of said impedance element, a capacity connected across said inductances, and condensers connected across said sections at the ends of said inductances, said inductances being'so related as to have mutual inductance.

21. A filter section having two pairs of terminals for connection to a line, shunt paths com-prisin condensers'between said pairs of termina s, inductances at both sides of one of said shunt paths, and a condenser connected across said inductances, an inductance at one side of one shunt path having mutual inductance with an inductance at opposite side, and said inductances and condensers being so proportioned that successive attenuation regions are produced, the attenuation region nearest zero frequency having successive frequencies of maximum attenuation. p

22. A irequehcy selective transmitting 1 network adapted to be. connected between two sections ofa circuit, said network cominductance.

23. A frequency selective transmitting network having two pairs of terminals for connection to a line, paths in shunt relation to said line and comprising condensers between said pairs of terminals, a reactive path from one terminal of one pair to one terminal of the other pair, said'path comprising inductance at one side of one of said shunt paths and inductance at the other side of said one shunt path, said inductances having mutual inductance and said reactive path comprising solely inductive reactance.

' In witness whereof, I hereunto subscribe my name tlllSdQtll day of December D.,

ROBERT C. MATHES. 

